U.S. patent number 6,362,615 [Application Number 09/386,937] was granted by the patent office on 2002-03-26 for electro-optic voltage sensor for sensing voltage in an e-field.
This patent grant is currently assigned to Bechtel BWXT Idaho LLC. Invention is credited to Thomas M. Crawford, James R. Davidson, Gary D. Seifert.
United States Patent |
6,362,615 |
Davidson , et al. |
March 26, 2002 |
Electro-optic voltage sensor for sensing voltage in an E-field
Abstract
A miniature electro-optic voltage sensor and system capable of
accurate operation at high voltages has a sensor body disposed in
an E-field. The body receives a source beam of electromagnetic
radiation. A polarization beam displacer separates the source light
beam into two beams with orthogonal linear polarizations. A wave
plate rotates the linear polarization to rotated polarization. A
transducer utilizes Pockels electro-optic effect and induces a
differential phase shift on the major and minor axes of the rotated
polarization in response to the E-field. A prism redirects the beam
back through the transducer, wave plate, and polarization beam
displacer. The prism also converts the rotated polarization to
circular or elliptical polarization. The wave plate rotates the
major and minor axes of the circular or elliptical polarization to
linear polarization. The polarization beam displacer separates the
beam into two beams of orthogonal linear polarization representing
the major and minor axes. The system may have a transmitter for
producing the beam of electro-magnetic radiation; a detector for
converting the two beams into electrical signals; and a signal
processor for determining the voltage.
Inventors: |
Davidson; James R. (Idaho
Falls, ID), Crawford; Thomas M. (Idaho Falls, ID),
Seifert; Gary D. (Idaho Falls, ID) |
Assignee: |
Bechtel BWXT Idaho LLC (Idaho
Falls, ID)
|
Family
ID: |
22270935 |
Appl.
No.: |
09/386,937 |
Filed: |
August 31, 1999 |
Current U.S.
Class: |
324/96;
324/754.27 |
Current CPC
Class: |
G01R
1/071 (20130101); G01R 15/242 (20130101) |
Current International
Class: |
G01R
15/24 (20060101); G01R 015/24 (); G02F
001/00 () |
Field of
Search: |
;324/750,751,752,96,244.1,501 ;250/227.23 ;356/370,368,400
;359/322,251,257 ;385/12 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Nguyen; Vinh P.
Attorney, Agent or Firm: Thorpe North & Western
Parent Case Text
CONTRACTUAL ORIGIN OF THE INVENTION
The present application claims priority from co-pending United
States Provisional Patent Application Ser. No. 60/098,794, entitled
"Electro-Optic Voltage Sensor," filed on Sep. 01, 1998.
Claims
We claim:
1. A electro-optic voltage sensor for sensing an E-field produced
by an energized conductor comprising: a sensor body configured for
disposition in the E-field and having an input configured for
receiving a source beam of electro-magnetic radiation within the
sensor body, the sensor body also having first and second outputs;
polarization beam displacer means disposed in the sensor body and
configured for separating the source beam into a first beam having
substantially a first linear polarization orientation and a second
beam having substantially a second linear polarization orientation,
and for directing the first beam along a first path and the second
beam along a different, second path; polarization altering means
disposed in the sensor body and configured for rotating the first
polarization of the first beam to a rotated polarization having
major and minor axes; sensing means disposed in the sensor body and
configured for inducing a differential phase shift on the major and
minor axes of the rotated polarization of the first beam in
response to the E-field; and redirecting means disposed in the
sensor body and configured for redirecting the first beam back
through at least the polarization altering means and polarization
beam displacer means; and wherein the polarization altering means
is configured for rotating the major and minor axes of the first
beam; and the polarization beam displacer means is configured for
separating the first beam into a third beam representing the major
axis of the polarized first beam and a fourth beam representing the
minor axis of the polarized first beam, the polarization beam
displacer means being configured for directing the third beam along
a third direction directly to the first output and the fourth beam
along a different, fourth direction directly to the second
output.
2. The sensor of claim 1, wherein the redirecting means converts
the rotated polarization of the first beam to circular or
elliptical polarization and further directs the first beam back
through the polarization altering means; and wherein the
polarization altering means is configured for rotating the major
and minor axes of the circular or elliptical polarization of the
first beam before the first beam enters the polarization beam
displacer means.
3. The sensor of claim 1, further comprising: collimator means
disposed in the sensor body configured for collimating the source
beam before the source beam enters the polarization beam displacer
means.
4. The sensor of claim 1, further comprising: collector means
disposed in the sensor body for collecting the third and fourth
beams.
5. The sensor of claim 1, further comprising: a collimator device
disposed in the sensor body at the input for collimating the source
beam, the collimator device having a sleeve, a fiber ferrule
disposed in the sleeve, and a graded index lens disposed in the
sleeve in contact with the fiber ferrule.
6. The sensor of claim 1, further comprising: a collector device
disposed in the sensor body at either the first or second output
for collecting either the third or fourth beams, the collector
device having a sleeve, a fiber ferrule disposed in the sleeve, and
a graded index lens disposed in the sleeve in contact with the
fiber ferrule.
7. A sensor of claim 1, wherein the polarization beam displacer
means further is configured for separating the third beam and the
fourth beam with a separation angle therebetween which is less than
90 degrees.
8. The sensor of claim 1, wherein the polarization beam displacer
means has opposite first and second surfaces, and wherein the
polarization beam displacer means is configured for receiving the
source beam at the first surface and passing the first beam through
the second surface, and for receiving the first beam back at the
second surface and passing the third and fourth beams at the first
surface.
9. The sensor of claim 1, wherein the sensor body configured for
disposition in the E-field produced between a conductor and a
grounded conductor without contacting the conductor.
10. An electro-optic voltage sensor for sensing an E-field produced
by an energized conductor comprising: a sensor body configured for
being disposed in the E-field without contacting the conductor and
having an input configured to receive a source beam of
electro-magnetic radiation and first and second outputs; a
collimator disposed in the sensor body and configured for
collimating the sorce beam; a polarization beam displacer disposed
in the sensor body and configured for separating the source beam
into a first beam of substantially a first polarization orientation
and a second beam of substantially a second orthogonal
polarization, and for directing the first beam along a first
direction and the second beam along a different, second direction;
polarization altering means disposed in the sensor body and
configured for rotating the first polarization of the first beam to
a rotated polarization; sensing means disposed in the sensor body
and configured for inducing a differential phase shift on the major
and minor axes of the rotated polarization of the first beam in
response to the E-field; and redirecting means disposed in the
sensor body and configured for redirecting the first beam back
through the sensing means, polarization altering means, and
polarization beam displacer, and for converting the rotated
polarization of the first beam to circular or elliptical
polarization; and wherein the sensing means is configured for
inducing a differential phase shift on the major and minor axes of
the circular or elliptical polarization of the first beam; the
polarization altering means is configured for rotating the major
and minor axes of the circular or elliptical polarization of the
first beam; and the polarization beam displacer is configured for
separating the first beam into a third beam representing the major
axis of the first beam and a fourth beam representing the minor
axis of the first beam, and for directing the third beam along a
third direction directly to the first output and the fourth beam
along a different, fourth direction directly to the second
output.
11. The sensor of claim 10, further comprising: first collector
means disposed at the first output and second collector means
disposed at the second output, the first and second collector means
being configured for collecting the third and fourth beams
respectively.
12. The sensor of claim 11, wherein the first and second collector
means each comprise a sleeve, a fiber ferrule disposed in the
sleeve, and a graded index lens disposed in the sleeve in contact
with the fiber ferrule.
13. A sensor of claim 10, wherein the collimator comprises a
sleeve, a fiber ferrule disposed in the sleeve, and a graded index
lens disposed in the sleeve in contact with the fiber ferrule.
14. A sensor of claim 10, wherein the polarization beam displacer
further is configured to separate the third beam and the fourth
beam with a separation angle therebetween which is less than 90
degrees.
15. A sensor of claim 10, wherein the polarization beam displacer
has opposite first and second surfaces, and wherein the
polarization beam displacer is configured for receiving the source
beam at the first surface and passing the first beam through the
second surface, and for receiving the first beam back at the second
surface and passing the third and fourth beams at the first
surface.
16. An electro-optic, voltage sensor system comprising: a conductor
and a grounded conductor configured for producing an E-field
therebetween when the conductor is energized; an elongated sensor
body disposed in the E-field between the conductor and the grounded
conductor without contacting the conductor, the sensor body having
first and second ends, the sensor body also having an input and
first and second outputs disposed at the first end of the sensor
body; a transmission source optically coupled to the sensor body
configured for producing a source beam of electro-magnetic
radiation, components of the source beam passing through the sensor
body defining a primary optical path, components of the source beam
passing from the first end to the second end defining an initial
pass, and from the second end to the first end defining a return
pass; a first fiber optic cable having a first end coupled to the
transmission source and a second end coupled to the input of the
sensor body for optically communicating the source beam from the
transmission source to the sensor body; a graded index lens
disposed in the sensor body at the first end and optically coupled
to the first fiber optic configured for collimating the source beam
as it passes through the lens on the initial pass; a polarization
beam displacer disposed in the sensor body and optically coupled to
the graded index lens configured for separating the source beam
into a first beam of substantially a first polarization orientation
and a second beam of substantially a second orthogonal
polarization, and for directing the first beam along a first
direction and the second beam along a different, second direction
as the source beam passes therethrough on the initial pass, the
first beam defining the primary optical path; a wave plate disposed
in the sensor body and optically coupled to the polarization beam
displacer configured for rotating the first polarization of the
first beam to a rotated polarization as the first beam passes
therethrough on the initial pass; a transducer disposed in the
sensor body and optically coupled to the wave plate configured for
inducing a differential phase shift on the major and minor axes of
the rotated polarization of the first beam in proportion to the
magnitude of the E-field as the first beam passes therethrough on
the initial pass; and a reflecting prism disposed in the sensor
body generally at the second end and optically coupled to the
transducer configured for reflecting the first beam back to the
first end of the sensor body defining the return pass and for
converting the rotated polarization of the first beam to circular
or elliptical polarization; and wherein the transducer is
configured for inducing a differential phase shift on the major and
minor axes of the circular or elliptical polarization of the first
beam as the first beam passes therethrough on the return pass; the
wave plate is configured for rotating the major and minor axes of
the circular or elliptical polarization of the first beam as the
first beam passes therethrough on the return pass; the polarization
beam displacer is configured for separating the first beam into a
third beam representing the major axis of the first beam and a
fourth beam representing the minor axis of the first beam, and for
directing the third beam along a third direction directly to the
first output and the fourth beam along a different, fourth
direction directly to the second output as the first beam passes
therethrough on the return pass; and further comprising a second
graded index lens disposed at the first output and a third graded
index lens disposed at the second output, the second and third
graded index lenses collecting the third and fourth beams
respectively.
17. A system of claim 16, wherein the polarization beam displacer
is configured for separating and directing the first and second
beams with a first separation angle therebetween, the first
separation angle being sized such that the second beam is directed
towards a side of the sensor body before reaching the reflecting
prism.
18. The system of claim 16, wherein the polarization beam displacer
is configured for separating and directing the third and fourth
beams with a second separation angle therebetween, the second
separation angle being less than 90 degrees.
19. The system of claim 16, wherein the polarization beam displacer
has opposite first and second surfaces and wherein the source beam
enters the polarization beam displacer at the first surface, the
first beam exits at the second surface on the initial pass and
enters at the second surface on the return pass, and the third and
fourth beams exit at the first surface.
20. The system of claim 16, further comprising: a detector
optically coupled to the sensor body by second and third fiber
optics coupled between the detector and the first and second
outputs, respectively, the detector comprising a first
photodetector optically coupled to the first fiber optic and a
second photodetector optically coupled to the second fiber optic,
for converting the third and fourth beams to electrical signals,
the detector further comprising a signal processor for determining
a desired E-field characteristic based on the electrical
signals.
21. The system of claim 16, wherein the transducer is a Pockels
crystal.
22. The system of claim 16, wherein the transducer a material, and
wherein the material is MgO-doped LiNbO.sub.3.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention The present invention relates generally
to an electro-optic high voltage sensor for sensing and/or
measuring an E-field produced by an energized conductor. More
particularly, it concerns an electro-optic voltage sensor which
utilized the Pockels electro-optic effect to measure voltage.
2. Background Art
High-accuracy measurement of high voltage has traditionally been
accomplished using iron-core ferro-magnetic potential transformers.
These devices have substantially limited dynamic range, bandwidth,
linearity, and electrical isolation. During electrical fault
conditions these transformers can conduct dangerous levels of fault
energy to downstream instrumentation and personnel, posing an
additional liability.
A variety of optic sensors for measuring voltage have been
developed in attempts to offer the power industry an alternative to
the conventional transformer technology. Generally, these voltage
sensor systems require that direct electrical contact be made with
the energized conductor. This contact is made necessary by the use
of a voltage divider which is utilized to connect the sensing
element with the energized conductor on which a measurement is to
be made. Direct electrical contact with the conductor may alter or
interrupt the operation of the power system by presenting a burden
or load.
In addition to the disadvantages associated with direct electrical
contact with the energized conductor, prior art voltage sensor
systems are typically bulky, particularly in extremely high voltage
applications. This is true because the size of the voltage divider
required is proportional to the voltage being measured. The size of
such systems can make them difficult and expensive to install and
house in substations.
Many prior art sensors are based upon the electrostrictive
principle which utilize interferometric modulation principles.
Unfortunately, interferometric modulation is extremely temperature
sensitive. This temperature sensitivity requires controlled
conditions in order to obtain accurate voltage measurements. The
requirement of controlled conditions limits the usefulness of such
systems and makes them unsuited for outdoor or uncontrolled
applications. In addition, interferometric modulation requires a
highly coherent source of electromagnetic radiation, which is
relatively expensive.
Open-air E-field based sensors have also been developed, but lack
accuracy when used for measuring voltage because the open-air
E-field used varies with many noisy parameters including ambient
dielectric constant, adjacent conductor voltages, moving conductive
structures such as passing vehicles, and other electromagnetic
noise contributions.
Systems which utilize mechanical modulation of the optical
reflection within an optic fiber have also been developed. Among
other drawbacks, the reliance of such systems on moving parts is a
significant deterrent to widespread use.
U.S. Pat. No. 5,892,357, issued Apr. 6, 1999, and assigned to the
same assignee of the present invention, discloses an electro-optic
voltage sensor which may be disposed in an E-field between an
energized conductor and a grounded conductor without contacting the
energized conductor. The electro-optic voltage sensor utilizes a
Pockles crystal or transducer which is sensitive to the E-field and
induces a differential phase shift on a beam of electro-magnetic
radiation traveling through the sensor in response to the E-field.
Although the electro-optic voltage sensor solves many of the
problems with the prior art, it still has some drawbacks. For
example, the electro-optic voltage sensor disclosed in the above
mentioned co-pending application utilizes a beam splitter to
separate orthogonal polarization components of the electro-magnetic
radiation. The beam splitter directs one component out of the
sensor in one direction, for example along a longitudinal axis of
the sensor, and directs the other component out a different
direction, perpendicular to the longitudinal axis of the sensor.
Therefore, either both components exit the sensor from different
sides, making the sensor difficult to locate between the conductor
and grounded conductor, or an additional reflector is required to
direct the other component so both components exit the same side,
making the sensor large.
It would therefore be an advantage in the art to provide a system
which does not require direct electrical contact with the energized
conductor, is compact, operates in a variety of temperatures and
conditions, is reliable, and is cost effective.
OBJECTS AND SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an
electro-optic voltage sensor system which does not require contact
with a conductor.
It is a flier object of the present invention to provide such an
electro-optic voltage sensor system which is capable of use in a
variety of environmental conditions.
It is a still further object of the present invention to provide
such an electro-optic voltage sensor system which can be employed
to accurately measure high voltages without use of dedicated
voltage division hardware.
It is an additional object of the present invention to provide such
an electro-optic voltage sensor system which minimizes the
electronics needed for implementation.
It is a further object of the present invention to provide a sensor
system capable of being integrated with existing types of high
voltage power transmission and distribution equipment so as to
reduce or eliminate the need for large stand-alone voltage
measurement apparatus.
It is yet another object of the present invention to provide a
sensor system capable of being integrated with existing types of
power transmission and distribution equipment.
It is yet another object of the present invention to provide a
sensor system with a sensor that is of small size.
While the present invention is described in terms of a sensor
system, it is to be understood that the subject apparatus and
method may be used in any field of electrical or optical
application. Those having ordinary skill in the field of this
invention will appreciate the advantages of the invention, and its
application to a wide variety of electrical uses.
The above objects and others not specifically recited are realized
in a specific illustrative embodiment of an electro-optical voltage
sensor device and system whereby one may measure the voltage
difference (or electrical potential difference) between objects or
positions. Voltage is a function of the electric field hereinafter
"electric field" shall be indicated "E-field") and the geometries,
compositions and distances of the conductive and insulating matter.
Where, as in the present invention, the effects of an E-field can
be observed, a voltage measurement can be calculated.
The sensor device may be utilized to sense or measure an E-field
using a source beam of electromagnetic radiation. The sensor device
comprises a sensor body disposed in the E-field. The sensor has an
input for receiving the source beam into the sensor body. The
sensor body also has first and second outputs.
A polarization beam displacer is disposed in the sensor body and is
optically coupled to the input. The polarization beam displacer
separates the source beam into a first beam having substantially a
first linear polarization orientation and a second beam having
substantially a second linear polarization orientation. The
polarization beam displacer also directs the first beam along a
first path and the second beam along a different second path. The
second beam may be discarded.
A wave plate is disposed in the sensor body and is optically
coupled to the polarization beam displacer for rotating the first
polarization of the first beam to a rotated polarization with major
and minor axes.
A transducer is disposed in the sensor body and is optically
coupled to the wave plate. The transducer induces a differential
phase shift on the major and minor axes of the rotated polarization
in response to the E-field when the transducer is exposed to the
E-field.
A reflecting prism is disposed in the sensor body and is optically
coupled to the transducer. The prism redirects the first beam back
through at least the polarization beam displacer means. The prism
may also reflect the first beam back through the transducer and
wave plate. The reflecting prism may also convert the rotated
polarization of the first beam to circular or elliptical
polarization.
The transducer may further induce a differential phase shift on the
major and minor axes of the circular or elliptical polarization of
the first beam as the first beam passes back therethrough. The wave
plate rotates the major and minor axes of the circular or
elliptical polarization of the first beam.
As the first beam passes back through the polarization beam
displacer, the polarization beam displacer separates the first beam
into a third beam representing the major axis of the first beam and
a fourth beam representing the minor axis of the first beam. The
polarization beam displacer also directs the third beam along a
third direction towards the first output and the fourth beam along
a different fourth direction towards the second output.
The invention may also comprise a graded index lens disposed in the
sensor body between the input and the polarization beam transducer.
The lens collimates the beam of electro-magnetic radiation. Other
lenses may also be used to collimate and/or collect the third and
fourth beams.
The invention may also comprise graded index lenses disposed in the
sensor body at the first and second outputs. The lenses collect and
focus the third and fourth beams.
The invention may also employ a transmitter, a detector, and a
signal processor. The transmitter produces a beam of
electro-magnetic radiation which is routed into the sensor device.
Although this electromagnetic radiation used in the present
invention can comprise any wavelengths beyond the visible spectrum,
the term "light", "beam", and/or "signal" may be used hereinafter
to denote electro-magnetic radiation for the purpose of
brevity.
The first beam undergoes an electro-optic effect when the sensor is
placed into the E-field, and is observable as a phase differential
shift of the major and minor axes of the elliptical polarization.
The planes of propagation are the object of the differential phase
shift. The differential phase shift causes a corresponding change
in the beam's polarization. The polarization change is in turn
converted into a set of amplitude modulated (AM) signals of
opposing polarity that are transmitted out of the sensor. The
detector converts the set of optical AM signals into electrical
signals from which the voltage is determined by the signal
processor.
The sensor processes the beam by splitting the beam in accordance
with the components of the orthogonal polarization planes into at
least two AM signals. These AM signals are then processed in an
analog circuit, a digital signal processor, or both. The AM signals
may be converted to digital signals, fed into a digital signal
processor and mathematically processed into a signal proportional
to the voltage which produced the E-field. In addition, the AM
signals may be optically processed. Alternatively, the output of
the analog circuit are a sinusoidal waveform representing the
frequency and peak-to-peak voltage and an RMS voltage. Additional
objects and advantages of the invention will be set forth in the
description which follows, and in part will be apparent from the
description, or maybe learned by the practice of the invention
without undue experimentation. The objects and advantages of the
invention may be realized and obtained by means of the instruments
and combinations particularly pointed out in the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the
invention will become apparent from a consideration of the
subsequent detailed description presented in connection with the
accompanying drawings in which:
FIG. 1 is a schematic view of a preferred embodiment an
electro-optic high-voltage sensor system in accordance with the
principles of the present invention;
FIG. 2 is a schematic view of a preferred embodiment of an
electro-optic voltage sensor device in accordance with the
principles of the present invention;
FIG. 3a is a side view of the preferred embodiment of the
electro-optic voltage sensor device in accordance with the
principles of the present invention showing a source beam of
electro-magnetic radiation on an initial pass through the
device;
FIG. 3b is a top view of the preferred embodiment of the
electro-optic voltage sensor device in accordance with the
principles of the present invention showing the source beam on the
initial pass and a return pass through the device;
FIG. 3c is a side view of the preferred embodiment of the
electro-optic voltage sensor device in accordance with the
principles of the present invention showing the source beam on the
return pass through the device;
FIG. 4 is an end view of the preferred embodiment of the
electro-optic voltage sensor device in accordance with the
principles of the present invention; and
FIG. 5 is a side view of a preferred embodiment of a coupling
between a fiber optic and the sensor device in accordance with the
principles of the present invention.
FIG. 6 is a diagram of the electro-optical voltage sensor system
configured to enable optical differentiation and summing of the
amplitudes of the components of the beam modified by the
E-field.
DETAILED DESCRIPTION OF THE INVENTION
For the purposes of promoting an understanding of the principles in
accordance with the invention, reference will now be made to the
embodiments illustrated in the drawings and specific language will
be used to describe the same. It will nevertheless be understood
that no limitation of the scope of the invention is thereby
intended. Any alterations and further modifications of the
inventive features illustrated herein, and any additional
applications of the principles of the invention as illustrated
herein which would normally occur to one skilled in the relevant
art and having possession of this disclosure, are to be considered
within the scope of the invention claimed.
As illustrated in FIG. 1, an electro-optic voltage sensor system,
indicated generally at 10, of the present invention is shown. The
system 10 may be used to sense and/or measure electrical
characteristics, such a voltage difference or electrical potential
difference, of an electrical device 14. The electrical device
produces an electrical field (indicated by arrows 18 in FIGS. 3a
and 3c) or E-field. The voltage is a function of the E-field (FIGS.
3a and 3c), and geometries, compositions, and distances of
conductive and insulating matter. Thus, where the effects an
E-field can be observed, a voltage measurement can be
calculated.
The electrical device 14 has a conductor 22, or an energized
conductor, and a grounded conductor 26. The electrical device may
have an insulator 30 between the energized conductor 22 and the
grounded conductor 26. The E-field (FIGS. 3a and 3c) may be
produced in the insulator 30 between the energized conductor 22 and
the grounded conductor 26 when voltage is applied to the conductor
22. The electrical device 14 may be a high-voltage electrical
cable, as shown. In the case of a high-voltage cable, the energized
conductor 22 is a high-voltage wire, the insulator 30 is insulation
or an insulated media surrounding the wire, and the grounded
conductor 26 is a sheath surrounding the wire and insulation. Thus,
the wire, insulation, and sheath are coaxial with a space between
the conductor and grounded conductor.
The high-voltage cable is one example of an electrical device or
E-field source. The electrical device may be any source of an
E-field, including for example, a shielded cable joint, a
terminator, a through-hole insulator, a shielded bus, an insulated
switchgear, or a duct-enclosed bus.
The system 10 includes a transmitter or transmitting source 34
which produces and transmits a source beam 38, as shown in FIGS.
2-3c, of electro-magnetic radiation. The transmitter 34 may produce
any wavelength of electro-magnetic radiation, including wavelengths
within the visible spectrum or beyond the visible spectrum. The
term "light", "beam", and/or "signal" may be used hereinafter to
denote all electro-magnetic radiation for the purpose of brevity.
The transmitter 34 may be a light source and the source beam 38 may
be a light beam produced by the light source. In the preferred
embodiment, laser light is used. Thus, the transmitter 34 may be a
laser and the source beam 38 may be the laser light. The
transmitting source 34 may be driven by drive electronics 36. The
drive electronics 36 may be controlled by a computer device
176.
The system 10 includes a sensor head or sensor device 40. The
sensor 40 is optically coupled to the transmitter 34 so that the
sensor 40 receives the source beam or light beam 38. A first fiber
optic or fiber optic cable 44 may be used to optically couple the
sensor 40 to the transmitter 34. The first fiber optic 44 has a
first end 46 coupled to the transmitter 34 and a second end 48
coupled to the sensor 40. The first fiber optic 44 directs or
optically communicates the source beam 38 from the transmitter 34
to the sensor 40. The fiber optic 44 electrically isolates the
sensor 40 from the transmitter 34 and the rest of the system to
protect personnel and equipment from the dangers of high voltage.
The first fiber optic 40 is preferably a single mode fiber.
The sensor 40 is advantageously disposed in the E-field (FIGS. 3a
and 3c) without contacting the energized conductor 22. The sensor
40 is preferably disposed between the energized conductor 22 and
the grounded conductor 26, or in the insulator 30. The sensor's 40
location and non-contact relationship to the conductor 40 also help
protect personnel and equipment form the dangers of high voltage.
Prior art sensors, or their transducers, directly connected to the
conductor by a capacitive voltage divider. The sensor 40 of the
present invention uses a fixed ground plane to partition the
E-field and does not require connection to the current carrying
conductor 22.
Referring to FIGS. 2 and 3a-c, the sensor 40 has an elongated
sensor body 52 with a first end 54 and a second end 56. The sensor
body 52 also may have a longitudinal axis 58. The first fiber optic
44 is coupled to the first end 54 of the sensor body 52, or the
source beam 38 is received by the sensor body 52 at the first end
54, defining an input 62. The body 52 also has first and second
outputs 64 and 65, as discussed more fully below. The source beam
38, or components thereof, passes through the sensor body 52,
preferably from the first end 54 to the second end 56 and back
again, defining a primary path 66. The source beam 38 preferably
passes from the first end 54 to the second end 56, defining an
initial pass 70. Thus, on the initial pass 70 the source beam 38
enters the sensor body 52. The source beam 38, or its components,
also preferably passes from the second end 56 to the first end 54,
defining a return pass 72. Thus, on the return pass 72 the source
beam 38, or its components, exit the sensor body 52. It is of
course understood that the source beam 38 need not pass is entirely
through the sensor body 52 to its physical end.
The sensor 40 has a graded index lens 76, or a collimator, disposed
in the sensor body 52 and optically coupled to the first fiber
optic 44, and thus the transmitter 34, so that the source beam 38
passes therethrough upon entering the sensor body 52. The graded
index lens 76 is preferably disposed at the first end 54 of the
sensor body 52 and is preferably the first optical element
encountered by the source beam 38 upon entering the sensor body 52.
The graded index lens 76 collimates the source beam 38 as the
source beam 38 passes through the lens 76 on the initial pass 70.
The graded index lens 76 is one example of a collimator means for
collimating the source beam 38. Any means for collimating the
source beam 38 may be used.
The sensor 40 advantageously has a polarization beam displacer 80
disposed in the sensor body 52 and optically coupled to the graded
index lens 76, or collimater. The polarization beam displacer 80 is
preferably disposed adjacent the graded index lens 76 and is
preferably the second optical element encountered by the source
beam 38 in the sensor body 52. It is of course understood that the
polarization beam displacer 80 need not be physically adjacent the
graded index lens 76 and may be separated therefrom by a space or
some translucent medium.
The polarization beam displacer 80 separates the source beam 38
into components, or first and second beams 84 and 86, as the source
beam 38 passes therethrough on the initial pass 70. The
polarization beam displacer 80 can be a piece of crystaline
birefringent material, such as calcite or equivalent, with a fast
and slow axis that provides for polarization separation of an
impinging light source, as known by those of skill in the art. The
source beam 38 has first and second orthogonal linear polarization
orientations (or horizontal and vertical polarization
orientations), indicated by arrows 90 and 92 respectively. The
source beam 38 may or may not be polarized upon arrival at the
polarization beam displacer 80, but the source beam 38 is
polarized, or re-polarized, by the polarization beam displacer 80.
The polarization beam displacer 80 separates the source beam into
two beams 84 and 86 of different, orthogonal linear polarization
orientations. Thus, the first beam 84 has substantially the first
linear polarization 90 (or vertical polarization) while the second
beam 86 has substantially the second linear polarization 92 (or
horizontal polarization).
In addition, the polarization beam displacer 80 directs the first
and second beams 84 and 86 along two different directions, or
different first and second paths 96 and 98. The first beam 84
traveling along the first path 96 defines the primary path 66. The
primary path 66 and first path 96 may be co-linear. The second beam
86 traveling along the second path 98 defines a secondary path. The
second beam 86 may be discarded by being directed out of, or
towards the sides, top or bottom of, the sensor body 52.
The first and second beams 84 and 86, or first and second paths 96
and 98, define a separation angle 100 therebetween. The
polarization beam displacer 80 directs the beams 84 and 86 in two
directions 96 and 98 with a relative angle, or separation angle
100. The separation angle 100 is preferably large enough that the
second beam 86 passes out of, or into, the top or bottom of the
sensor body 52 without the second beam 86 reaching the second end
56 of the sensor body 52, and is thus discarded. The separation
angle 100 and length of the sensor body 52 are related. A larger
separation angle 100 results in a shorter sensor body 52. A smaller
separation angle 100 results in a longer sensor body 52. Because
the sensor 40 may be disposed in an insulator 30, between a
conductor and grounded conductor 26, as shown in FIG. 1, which may
be a narrow space, the sensor body 52 is preferably narrow.
The polarization beam displacer 80 has opposite first and second
surfaces 101 and 102, or first and second ends. The polarization
beam displacer 80 receives the source beam 38 at, or through, the
first surface 101 and passes the first beam 84 through the second
surface 102. The polarization beam displacer 80 receives the first
beam 84 back at, or through, the second surface 102.
The polarization beam displacer 80 is one example of a means for
separating the source beam 38 into a first beam 84 of the first
polarization 90 and a second beam 86 of the second polarization 92,
and for directing the first and second beams 84 and 86 along
different paths or in different directions. Any such means for
separating and directing the beams may be used which preferably
separates the beams by less than 90 degrees.
The sensor 40 includes a half wave plate 104 disposed in the sensor
body 52 and optically coupled to the polarization beam displacer
80. The wave plate 104 is preferably disposed adjacent the
polarization beam displacer 80 and is preferably the third optical
element encountered by the source beam 38, or its component the
first beam 84. Again, it is understood that the wave plate 104 need
not contact the polarization beam displacer 80, but may be
separated therefrom by a space or translucent medium. The wave
plate 104 rotates the first polarization 90 of the first beam 84,
indicated at 108, as the first beam 84 passes through the wave
plate 104 on the initial pass 70. The wave plate 104 is preferably
a half wave plate oriented to rotate the polarization 45 degrees
with respect to the electric field on the transducer, as discussed
below. The wave plate 104 is one example of a polarization altering
means for rotating the linear polarized beams, and circular or
elliptical polarized beams. Any such means for converting or
rotating the polarization of the beams may be used, including for
example, any wave retardation optic or combination of optics.
The sensor 40 also includes a cell or transducer 114 disposed in
the sensor body 52 and optically coupled to the wave plate 104. The
cell 114 is preferably disposed adjacent the wave plate 104 and is
preferably the fourth optical element encountered by the source
beam 38, or its component the first beam 84. It is understood that
the cell 114 need not be in contact with the polarization beam
displacer which is responsive to the E-field 18. The cell 114 or
material alters the polarization 108 of the first beam 84, or major
and minor axes of the rotated polarization 108, in response to the
E-field 18, and in proportion to the magnitude of the E-field 18,
as the first beam 84 passes therethrough on the initial pass 70.
Thus, the cell 104 may be a Pockels crystal or Pockels transducer.
The cell 114 or material is preferably MgO-doped LiNbO.sub.3. In
addition, the MgO-doped LiNbO.sub.3 material is preferably
z-cut.
The sensor 40 includes a reflecting prism 130 disposed in the
sensor body 52 and optically coupled to the cell or transducer 114.
The reflecting prism 130 is preferably disposed at the second end
56 of the sensor body 52 and is preferably the fifth optical
element encountered by the source beam 38 or its component the
first beam 84. Again, it is understood that the prism 130 need not
contact the cell 144. It is also understood that the prism 130 need
not be disposed at the physical end of the sensor body 52, but
preferably defines the end of the sensor body with respect to the
source beam 38 or its components. The reflecting prism 130 reflects
the first beam 84 back to the first end 54 of the sensor body 52
defining the return path 72. In addition, the reflecting prism 130
preferably reflects the source beam 38, or its component the first
beam 84, back through the cell 114, the wave plate 104, the
polarization beam displacer 80, and the graded index lens 76.
The reflecting prism 130 is one example of reflecting means for
reflecting the first beam 84 back through the body 52. The first
beam 84 on the return pass 72 travels substantially parallel to the
first beam 84 on the initial pass 70. By reflecting or redirecting
the beam 84 back towards the first end 54 of the sensor body 52,
the beam may exit the same end it entered. Any means for returning
the beam of electro-magnetic radiation back through the body may be
used, including for example, a mirror, a light guide, a fiber
optic, etc.
The reflecting prism 130 also preferably converts the rotated
polarization 108 of the first beam 84 to circular or elliptical
polarization, indicated at 118. In the absence of an electric field
across the transducer 114, the transducer will not induce a phase
shift on the major and minor axes and the reflecting prism 130 will
convert the rotated polarization 108 to circular polarization. If
an electric field is present across the transducer 114, the
transducer will induce a phase shift on the major and minor axes
and the reflecting prism 130 will convert the rotated polarization
108 to elliptical polarization.
As the first beam 84 again passes through the cell or transducer
114 on the return pass 72, the cell 114 induces a differential
phase shift on the major and minor axes of the circular or
elliptical polarization 118 of the first beam 84. The return pass
72 is in the opposite direction of the y-axis and helps to
compensate for temperature induced bi-refringence.
When the cell or transducer 114 (also called the transducing
medium) is in a non-zero E-field (not shown) it induces a
"differential phase shift" to orthogonal beam components of a beam
through the Pockels electro-optic effect, which will now be
explained. In the polarized beam the light has at least two
components which propagate along at least two orthogonal planes,
respectively, thus forming at least two orthogonal planes within
the beam. The phase of the components in each plane of propagation
are the object of a shift, relative to the phase of the component
in the other plane, in the transducer. The Pockels electro-optic
effect, which takes place in the transducer, changes the relative
phases of the beam components by altering their respective
velocities, and is observed in Pockels transducing crystals and
similar media. The magnitude of the phase shift, called the
"differential phase shift", is proportional to the magnitude of the
E-field. Thus, the Pockels electro-optic effect is observed as a
"phase differential shift" of the orthogonal beam components which
is proportional to the magnitude of the E-field. Due to the fixed
coaxial structure of the devices in which the sensor head is to be
installed, the magnitude of the E-field is proportional to the
voltage. Therefore, the differential phase shift is proportional to
(and can be used to measure) the voltage of the E-field between
energized conductor and ground conductor.
The phase shift between orthogonal components further manifests
itself as an alteration of the beam's polarization. Therefore, the
beam may be considered either to be a differential phase shifted
signal or an optical polarization modulation signal. The
polarization modulation signal is used in the present invention
because it can be detected using low-cost, components that are less
susceptible to temperature, mechanical perturbations, and optical
incoherence than those components that would be required to sense
the differential phase shift directly.
In the practice of the present invention, the sensor 40, the sensor
body 52, or the transducer 114 may be encased in a dielectric
buffering material, not shown, to smooth the transition geometry
between the permittivity of the transducer 114 and the permittivity
of the surrounding media, which in most cases will be an insulator.
The dielectric buffering material promotes uniformity in the
E-field, particularly around the edges of the transducer 114. This
enhances uniform phase shift in the beam passing through the
transducer 114, and minimizes voltage stress on the materials in
and surrounding the sensor 40 as well, thereby increasing the
probable maximum operating lifetime of the entire system.
As the first beam 84 again passes through the wave plate 104 on the
return pass 72, the wave plate 104 rotates the major and minor axes
of the circular or elliptical polarization 118 of the first beam 84
as the first beam 84 passes therethrough, representing the major
and minor axes of the ellipse. The wave plate 104 rotates the major
and minor axes of the beam 45 degrees to align ellipse axes to the
beam separator 80, or so that the major and minor axes are coplanar
with the beam displacer 80.
The polarization beam displacer 80 separates the first beam 84 into
a third beam 140 and a fourth beam 142. The third beam 140 has the
first linear polarization 90 while the fourth beam 142 has the
second linear polarization 92. The third beam represents the major
axis of the elliptical polarization 118 while the fourth beam
represents the minor axis of the elliptical polarization 118. Thus,
the beam displacer 80 converts the ellipse into two amplitude
signals.
In addition, the polarization beam displacer 80 directs the third
and fourth beams 140 and 42 along two different directions, or
different third and fourth paths 150 and 152. The polarization beam
displacer 80 directs the third beam 140 towards the first output
64, and the fourth beam 142 towards the second output 65.
The third and fourth beams 140 and 142, or third and fourth paths
150 and 152, define a separation angle 156 therebetween. The
polarization beam displacer 80 directs the beams 140 and 142 in two
directions with a relative angle, or separation angle 156. The
separation angle 156 is preferably large enough that the third and
fourth beams 140 and 142 become separated in a relatively short
distance with respect to the sensor body 52. In addition, the
separation angle 156 is preferably small enough that the sensor
body 52 remains relatively narrow or slender. A larger separation
angle 156 results in a shorter sensor body 52, while a smaller
separation angle 156 results in a longer sensor body 52.he sensor
body 52 are related. Because the sensor 40 may be disposed in an
insulator 30, between a conductor 22 and grounded conductor 26, as
shown in FIG. 1, which may be a narrow space, the sensor body 52 is
preferably narrow, or the separation angle 156 is preferably
small.
The separation angle 156 advantageously is preferably at least less
than an angle separating two beams from a beam splitter, or less
than 90 degrees. In addition, the polarization beam displacer 80
advantageously passes the third and fourth beams 140 and 142
through the first surface 101. Therefore, the input 62 and first
and second outputs 64 and 65 are advantageously located at the same
end of the sensor 40, the first end 54 of the sensor body 52, to
facilitate insertion of the sensor 40 in the E-field between the
conductor 22 and grounded conductor 26. In addition, the source
beam 38 preferably enters the sensor body 52 and the third and
fourth beams 140 and 142 preferably exit the sensor body 52 in a
straight linear manner, without right angles, to minimize the size
of the sensor 40. It should be noted that a graded index lens
allows off-axis beam collection in the deflected signal, as
discussed more fully below.
Prior art beam splitters, on the other hand, typically separate
beams so that one remains relatively straight while the other is
directed at a right angle, or perpendicular, to the first. In
addition, such beam splitters typically pass one of the beams out
of side or plane perpendicular to a side or plane in which the beam
enters. Such beam splitters require an additional mirror or prism
to redirect the perpendicular beam to be more parallel with the
first. The perpendicular beam and addition prism increase the width
of the prior art sensor. Although the beam splitter of the prior
art may be removed from the prior art sensor head itself and
located elsewhere in the optical path, for example prior to the
detector, such a solution requires additional components.
In the polarizing beam displacer 80, the first beam 84 is separated
in accordance with the respective axes of its polarization ellipse
118 into amplitude modulated (AM) signals, or third and fourth
beams 140 and 142 with first and second linear polarizations 90 and
92, respectively. The said polarization ellipse 118 will exhibit an
ellipticity ranging between -1 and +1, in proportion to voltage at
any given time. Those skilled in the art will note that an elliptic
polarization whose ellipticity ranges between -1 and +1 can be
described as ranging from a linear polarization along one axis, for
example 90, to a linear polarization along a second axis, for
example 92, perpendicular to the first axis, wherein the point at
which ellipticity equals 0 corresponds to circular polarization.
The major and minor axes of the polarization ellipse 118 of the
first beam 84 can be represented by two orthogonal components, or
third and fourth beams 140 and 142 with first and second
polarizations 90 and 92, respectively. The polarization beam
displacer 80 then separates the first beam 84 into two components,
or third and fourth beams 140 and 142 with first and second
polarizations 90 and 92, respectively, which comprise the
intensities along each of the two axes of the polarization ellipse
118 shown as orthogonal components 90 and 92. The intensity of beam
components, or the third and fourth beams 140 and 142, will
modulate conversely to one another in response to modulations in
the ellipticity of the beam's polarization. The beam components are
two amplitude modulated (AM) signals, shown as 140 and 142,
respectively.
The third and fourth beams 140 and 142 may be passed through second
and third graded index lenses 157 and 158 which collect the third
and fourth beams 140 and 143 and couple them to the outputs or
fiber optic by focusing the beams into the fiber optic. The third
and fourth beams 140 and 142 may be passed through separate beam
collection optics, different from the collimator 76 which
collimates the source beam 38, as discussed more fully below.
Referring to FIG. 1, the sensor system includes a detector 160 for
receiving the two AM signals, or third and fourth beams 140 and
142. The detector 160 may include first and second photo-detectors
162 and 164 for receiving the AM signals 140 and 142, respectively.
The third and fourth beams 140 and 142 maybe optically
communicated, or the sensor 40 optically coupled, to the first and
second photo-detectors 162 and 164 by second and third fiber optics
166 and 168, respectively. The fiber optics 166 and 168 are coupled
at one end to the sensor body 52, at the first and second outputs
64 and 65, respectively, and at another end to the first and second
photo-detectors 162 and 164. In the preferred embodiment the second
and third fiber optics 166 and 168 comprise at least one optic
fiber, wherein the optic fiber is a multi-mode optic fiber.
In the photo-detectors 162 and 164, the AM signals 140 and 142
become electrical signals. The electrical signals are routed into a
signal processor 174, which may be part of the detector 160,
wherein a desired E-field characteristic is determined,
particularly that of voltage. In addition, the signals may be
routed to a computer device 176. The AM signals may be processed by
an analog circuit, a digital signal processor, or a combination of
both.
To determine the voltage in the practice of the preferred
embodiment of the present invention the signal processor 174, or
computer 176, is designed to process each AM signal, in an analog
circuit, digital signal processor, or both. The digital signal
processor receives the AM signals when converted into digital
signals and mathematically processes them into a signal
proportional to the voltage which produced the E-field. In
addition, the AM signals may also be optically processed, as
discussed further below. Furthermore, the outputs of the analog
circuit are a sinusoidal waveform representing the frequency and
peak-to-peak voltage and RMS voltage, as discussed further below.
The signal processor or computer produces a display signal (not
shown) which is then displayed on a readable display 178 such as:
digital, hardcopy, video, software, computer memory displays or an
audible indicator.
While it is possible to actually measure the relative phases of the
orthogonal components 90 and 92 of the beams 140 and 142 after
exiting the transducer 144, the relative phase shift can also be
derived from the intensities of the AM signals 140 and 142 without
using complex and costly approaches as involved in direct phase
measurements. Therefore, in the present invention, when the two AM
signals 140 and 142 are separated from a single differential phase
shifted signal 84 using the polarizing beam displacer 80, the
beam's polarization state is analyzed to obtain AM intensity
signals. The AM signals 140 and 142 extracted from the beam's
polarization state by the polarizing beam displacer 80 are
transmitted to and used in the signal processor 174 where their
complementary nature facilitates rejection of common mode noise and
minimizes effects of temperature dependent intrinsic birefringence
that may reside in the transducing medium or other optical
components within the system. This feature of the present invention
substantially enhances accuracy and practicality of the system and
represents an additional advancement over much of the prior art.
The signal processor 174 performs these functions while converting
the received AM signals 140 and 142 into a single, highly accurate
voltage measurement. In addition to measuring the voltage of a
device, the invention may be used in conjunction with a device for
measuring current to provide information regarding power, power
factor angle, and energy on the conductor of interest.
As mentioned, each AM signal 140 and 142 is converted by a
photo-detector 162 and 164 into a electrical signal which can be
processed by the signal processor 174. The photo-detector comprises
an optic-to-electronic conversion means for converting the AM
signals into analog electronic signals. Preferably, the analog
electronic signals comprise low-level analog voltage signals or
current signals.
In the preferred embodiment of the present invention the electrical
signals are electronic signals transmitted to the signal processor
174 which correspond to the intensity of the AM signals 140 and
142. Thus, in the practice of the present invention, a series of AM
signals are manipulated by the signal processor 174, as each of the
electrical signals corresponds to intensity of each AM signal 140
and 142. The electrical signals may be sampled by the signal
processor 174 at regular intervals and substantially simultaneously
with one another. The sampled signals are the instantaneous
intensity for each AM signal 140 and 142. The signals will be
discussed below as (A) and (B), respectively.
In the signal processor 174 the instantaneous intensity signal for
each beam component 140 and 142 is sampled sequentially and stored,
thereby forming a data base of stored signals which represents each
AM signal over time. The stored signals are then converted into a
displayable signal regarding the voltage of E-field at the
transducer 114.
In the preferred embodiment signals are manipulated in the
following manner. Referring to FIG. 6, in accordance with the
principles of the present invention, it has been discovered that
the optical signals produced can be summed and differentiated in a
rapid and inexpensive manner by avoiding high speed digital
processing, and relying on summing and differentiating of the
actual optical signal. In that the polarized light 84 contains two
components 90 and 92 (FIG. 3c), hereinafter referred to component A
and component B for convenience, it has been found that selectively
combining and differentiating of component A and component B can
result in inexpensive, rapid and highly accurate voltage
determination.
Component A and component B are separated by the beam displacer 80
(FIG. 3c). Component A of the phase shifted light is the sent
through optical fiber 166 and component B of the phase shifted
light is sent through optical fiber 168. A 50:50 beam splitter 300
is disposed along optical fiber 166 to divide component A into two
beams, each having equal intensity. The two beams of component A
are carried by optical coupling means 166a and 166b respectively.
The optical coupling means 166a and 166b maybe mirrors,
multiple-mode optical fibers, light pipes, relay optics or any
other means for transmitting the light in the manner discussed
herein.
Likewise, component B of the phase shifted light is divided into
two beams of equal intensity by being passed through a 50:50 beam
splitter 304. The two beams of component B are then sent through
optical coupling means 168a and 168b, which will typically be in
the same form as optical coupling means 166a and 168a. For the sake
of discussion, component A is that portion of the phase shifted
beam 84 which is propagated parallel to the E-field, and component
B is that portion of the beam which is propagated perpendicular to
the E-field.
In order to differentiate components A and B, a 1/4.lambda. plate
308 is placed along optic fiber 168a. Component B carried by optic
fiber 168a is then combined with component A carried by optic fiber
166b. Because of the 1/4.lambda. plate 308, component A and
component B are 180 degrees out of phase. Since the beams are 180
degrees out of phase, the two beams will subtract from each other
when they are combined, as represented by box 312. This, in turn,
produces a difference signal (A-B), carried by an optical means,
such as a multi-mode optic fiber 316.
In order to sum the components A and B, a 1/4.lambda. plate 320 is
placed along optic coupling means 166a to change the phase of
component A. Because of the 1/4.lambda. plate 320, component beam A
and component beam B are in phase when the two beams are combined,
as represented by box 324. Since the two beams are in phase, the
two beams will add to one another when combined, thereby producing
a sum signal (A+B), carried by optical means, such as a multi-mode
fiber 328.
Once the difference signal and the sum signal are determined, the
elliptical-polarization of the beam can be readily determined as
the difference divided by the sum. ##EQU1##
wherein .theta. is the phase difference between component A and
component B. Of course, the phase difference is proportional to the
E-field, which is proportional to the voltage.
One significant advantage of the configuration shown in FIG. 6 is
that it decreases the effect of misalignments. By determining
intensity in the manner described, first order misalignments cancel
out. Thus, a less precision is required in manufacturing the sensor
while still facilitating the ability to obtain a highly accurate
determination of voltage. This, in turn, reduces manufacturing
costs and results in fewer errors.
The above method provides an optical solution to creating sum and
difference signals for post phase rotation analysis. The signals
are created at optical speeds without the need for fast electronics
to produce the information. Additionally, this optical solution
also simplifies problems created by unmatched photodiode response
characteristics. Calibration between the two photodiodes now
requires only simple gain changes. Likewise, errors induced by
changes in the transmission characteristics of the optical fibers
are reduced.
As indicated above, the AM signals also may be processed by an
analog circuit. The analog circuit would essentially accomplish the
same function as the optical system described in FIG. 6. The analog
circuit may sum the signals and difference the signals. The analog
circuit may then determine the inverse sin of the ratio of the
difference and sum signals. Using a scaling factor the peak-to-peak
voltage may be determined. In addition, the RMS voltage may be
extracted.
The detector 160 and signal processor 174 may be combined into a
single unit. Likewise, the transmission source 34 and drive
electronics 36 may be combined into a single unit. As shown in FIG.
1, all the components, such as the detector 160, the signal
processor 174, the transmission source 34, and the drive
electronics 36 may be a single unit 184.
Referring to FIG. 5, the first optical fiber 44, may be coupled to
the graded index lens 76 by a collimator device 190. The collimator
device 190 has a standard fiber ferrule 192 contacting the graded
index lens 76. The fiber optic 44 is mounted in the fiber ferrule
192. The fiber ferrule 192 and graded index lens 76 are disposed in
a connecting sleeve 194. Therefore, the source beam 38 exiting the
collimator device 190 is collimated.
First and second collector devices 198 with similar structure may
also be used for the outputs 64 and 65, as shown in FIG. 4, where
the second and third graded index lenses 157 and 158 collect the
third and fourth beams. Thus, the source beam 38 may be collimated
by a first collector device 198 with a first graded index lens 76,
while the third and fourth beams 140 and 142 are collected by first
and second collector devices each with a separate graded index
lens. As the third and fourth beams 140 and 142 enter similar
collector devices 198 the graded index lenses 157 and 158 act as a
beam collectors due to their ability to accept off-axis beams and
concentrate the beams into the fiber core. This ability simplifies
alignment during assembly and allows improved collection from the
displaced output beams 140 and 142. The collimator device 190 and
collector devices 198 may be disposed in the sensor body 52 or form
part of the sensor body 52.
Referring to FIG. 4, the collimator device 190 and collector
devices 198 are housed in a cell or housing 200. The cell 200
receives the collimator device 190 and collector devices 198 as
well as at least a portion of the polarization beam displacer 80 to
align the input 62 and outputs 64 and 65 with the displacer 80. A
dummy collimator 202 may be used as a placeholder to allow for easy
alignment. The dummy collimator 202 may also extend beyond the
housing 200 to act as a strain relief for the other fibers 44, 166
and 168. The collimator device 190 is an example of collimator
means for collimating the source beam 38, and the collector device
198 is an example of collector means for collecting the third and
fourth beams 140 and 142.
In an alternative, the signals may be manipulated in the following
manner. First, an average intensity for each independent amplitude
modulated signal is calculated. This is done by summing the
instantaneous intensities of the signals which have been sampled
over a pre-determined time interval and dividing by the number of
samples taken in the interval. In the preferred invention this is
accomplished by taking the average of the signals over time for
each beam component by summing the signals of each beam component
and dividing the sum by the number of signal samples taken.
Mathematically, the average intensity for the AM signal (A) is
expressed as follows: ##EQU2##
where the average intensity is (A), the instantaneous AM signal is
(A.sub.i), the number of samples is (L), the samples are taken and
stored at uniform time intervals (i), and the average is calculated
using samples between present time index n and past time index
(n-L). Similarly, the average intensity for the AM signal (B) is
expressed as follows: ##EQU3##
where the average intensity is (B), the instantaneous AM signal is
(B.sub.i), with the other values being as above.
Next, an adjusted instantaneous intensity for each signal is found
by comparing the most recent instantaneous signal intensity with
the average signal intensity of the corresponding AM signal. Thus,
for the beam component corresponding to AM signal (A), the adjusted
instantaneous intensity (.alpha..sub.n) is:
Where at (A.sub.n) is the instantaneous intensity of AM signal (A)
at the present time index. Similarly, for AM signal (B), the
adjusted instantaneous intensity (.beta..sub.n) is:
Where (B.sub.n) is the instantaneous intensity of signal (B) at the
present time. It will be recognized by those skilled in the art
that because signals (A) and (B) each represents a different axis
on the polarization ellipse, their amplitudes will change in
opposite directions from one another for a given change in
polarization. Thus where the intensity of one signal increases
there will be a decrease of intensity of equal magnitude in the
other signal. Therefore, the adjusted instantaneous intensity
signals (.alpha..sub.n) and (.beta..sub.n) must be computed as
indicated above in order to preserve sign.
The adjusted average instantaneous intensity signal for both
signals (A) and (B) compensates for any temperature induced
birefringence that may exist within the transducer. Temperature
induced birefringence causes a change in the intensity of the AM
signals over time, as the transducer heat or cools. The variation
in the intensity due to temperature-dependant intrinsic
birefringence of the transducer appears as a modulation or
variation in the average intensity. Thus, by comparing the
instantaneous intensity with the average intensity of the signals,
and deducting the average intensity from the instantaneous
intensity, temperature induced variations of the signal due to the
birefringence in the transducer are compensated for in the adjusted
instantaneous intensity signals (.alpha..sub.n) and
(.beta..sub.n).
An additional manipulation of the adjusted instantaneous intensity
signals (.alpha..sub.n) and (.beta..sub.n) compensates for
intensity fluctuations and other common mode noise. This is
accomplished by comparing the average of the adjusted instantaneous
intensity signals (.alpha..sub.i) and (.beta..sub.i) for the
signals (A) and (B). This comparison entails calculating the
average between (.alpha..sub.n) and the sign-reversed value of
(.beta..sub.n). ##EQU4##
This average (.gamma..sub.n) is directly proportional to the
voltage. This is so because the Pockels electro-optic effect
induces a differential phase shift in the orthogonal planes 90 and
92 of the beams 140 and 142 which is directly proportional to the
E-field 18, and the E-field is directly proportional to voltage.
Thus, for a sampling of interest (n), the average instantaneous
intensity signal (.gamma..sub.n) for the signals (A) and (B) is
directly proportional to the actual instantaneous voltage (V.sub.n)
between energized conductor and ground, varying only by a scaling
constant (K). ##EQU5##
The scaling constant (K) is determined by applying a precisely
known voltage to the device of interest and adjusting the scaling
constant (K) until the value measured as the actual instantaneous
voltage (V.sub.n) is equivalent to the precisely known voltage
being applied. In a typical general application of the present
invention, shown in FIG. 1, the sensor head 40 is placed in an
insulator 30 between a conductor and a grounded conductor 26. When
voltage is applied to the conductor an E-field (FIGS. 3a and 3c) is
created between the conductor 22 and the grounded conductor 26, in
the insulator 30. Determination of the scaling constant (K) is
accomplished by applying a precisely known voltage to the conductor
22. Once the scaling constant (K) is known the electro-optical
voltage sensor system may be operated to determine additional
actual instantaneous voltages applied to conductor 22.
Although the optical elements described above, such as the
polarization beam displacer 80, the wave plate 104, and the cell or
transducer 144, each have been described as single units, or as
single optical elements, with the source beam 38, and its component
the first beam 84, passing through each optical element twice, a
first time on the initial pass 70 and a second time on the return
pass 72, it is of course understood that the optical elements may
comprise multiple units, such as first and second optical elements,
with the source beam 38, or its components such as the first beam
84, passing through a first optical unit on the initial pass 70 and
a second optical unit on the return pass 72. For example, the
polarization beam displacer 80 may comprise a first displacer and a
second displacer, with the source beam 38 passing through the first
displacer on the initial pass and with the first beam 84 passing
through the second displacer on the return pass 72. Similarly, the
wave plate 104 and transducer 144 may also be comprised of first
and second units.
In addition, although the first beam 84 has been described as
returning through all the optical elements (the displacer 80, the
wave plate 104, and transducer 144) on the return pass 72, it is of
course understood that the optical elements may be configured so
that the first beam 84 passes through less than all of the optical
elements on the return pass 82 or the initial pass. For example,
the first beam 84 may not pass through the transducer 144 on the
return pass 72. Similarly, the optical elements may be configured
such the source beam 38 passes through less than all of the optical
elements. For example, the source beam 38 may not pass through the
transducer on the initial pass 72. As another example, the source
beam 38 may not pass through the polarization beam displacer 80 if
the source beam 38 has already been separated into first and
secohogonal polarization orientations.
It is to be understood that the above-described arrangements are
only illustrative of the application of the principles of the
present invention. Numerous modifications and alternative
arrangements may be devised by those skilled in the art without
departing from the spirit and scope of the present invention and
the appended claims are intended to cover such modifications and
arrangements. Thus, while the present invention has been shown in
the drawings and fully described above with particularity and
detail in connection with what is presently deemed to be the most
practical and preferred embodiment(s) of the invention, it will be
apparent to those of ordinary skill in the art that numerous
modifications, including, but not limited to, variations in size,
materials, shape, form, function and manner of operation, assembly
and use may be made without departing from the principles and
concepts set forth herein.
* * * * *